The invention relates to a process for structuring a photoresist layer.
In semiconductor technology, photolithographic processes for producing integrated circuits on a semiconductor substrate play a key part. Typically in such processes, photoresist layers are applied to the surface of the substrate that is to be structured or patterned and are then patterningly exposed to radiation of an appropriate wavelength range. Patterning exposure takes place by a lithography mask that determines the structure that is to be transferred into the substrate. The exposed regions of the photoresist layer are chemically modified by exposure and, as a result, change their polarity. The exposed and unexposed regions of the photoresist, therefore, present different solubilities to an appropriate developer. In the subsequent developing step, this fact is utilized for the selective removal of the exposed or unexposed areas. The areas of the photoresist layer that remain on the substrate act, in the subsequent structuring step, as a mask that protects the underlying substrate layer against removal of material or a change in material. Such a structuring step may include, for example, plasma etching, wet chemical etching, or ion implantation.
Particularly well-established, both for the one-layer resists developable under wet conditions and for the two-layer resist systems that can be completely or partly developed under dry conditions, are chemically reinforced resists (chemical amplification resists; CAR). A characteristic feature of chemical amplification resists is that they include a photoacid generator, i.e., a photosensitive compound that, on exposure to light, generates a protic acid. Such protic acid, where appropriate with thermal treatment of the resist, then initiates acid-catalyzed reactions in the base polymer of the resist. As a result of the presence of the photoacid generator, the sensitivity of the photoresist is substantially increased as compared with that of a conventional photoresist. An overview of this topic is given by H. Ito in Solid State Technology, July 1996 p. 164 ff.
In the case of the positive resists, the different solubility of the exposed and unexposed photoresist is achieved by the principle of acid-catalyzed cleavage. Starting from an apolar chemical group of the layer-forming polymer, e.g., a tert-butyl carboxylate group, a polar carboxylic acid group is formed in the presence of a photolytically generated acid, where appropriate in a heating step. Further examples of apolar xe2x80x9cblockedxe2x80x9d groups that can be converted into corresponding polar groups by acid-catalyzed reactions are the tert-butoxycarbonyloxy (t-BOC) group or acetal groups. Through the conversion of the apolar group into the corresponding polar group, the resist undergoes a change in polarity in the previously irradiated areas, and, as a result, becomes soluble in the polar, aqueous-alkaline developer. Consequently, the developer may selectively remove exposed areas of the photoresist. Accordingly, the resist residues in the unexposed, apolar areas geometrically define a resist profile or resist pattern on the substrate, which serves as a mask for surface structuring in subsequent process steps.
In negative resists, in contrast, exposure brings about a reduction in the solubility of the photoresist in the exposed areas. To achieve this, negative-working photoresists generally contain crosslinkable groups that are able to undergo crosslinking reactions under the influence of irradiation. The crosslinking decreases the solubility of the exposed areas of the photoresist in a corresponding developer. The crosslinkable groups may either be attached directly to the base polymer or be present as a separate crosslinking component in the photoresist. In negative-working chemical amplification resists, groups crosslinkable by acid catalysis are used that are activated by the photolytically liberated acid.
Due to the constantly increasing integration density in semiconductor technology, the accuracy with which the resist profile can be produced after development on a surface to be structured is of critical significance. The resist profile is, on one hand, physically uniquely predefined by the light distribution during exposure. On the other hand, it is chemically transferred into the resist layer by the distribution of the components generated photochemically by the exposure process.
Due to the physicochemical properties of the resist materials, completely unfalsified transfer of the pattern predetermined by the lithography mask to the resist is, however, not unambiguously possible. In particular, interference effects and light scattering in the photoresist play a major part here. However, the steps following exposure, such as development, for example, also have a great effect on the quality of the resist profiles, which is determined substantially by the profile sidewalls. To achieve surface structuring that is as precise as possible in the subsequent process steps, it would be ideal if it were possible to obtain virtually perpendicular, smooth profile sidewalls in the resist profile after the development of the photoresist.
The light intensity profile that is established in the course of exposure in the photoresist, in particular, has a negative impact on the achievable steepness of the profile sidewalls. The characteristic intensity profile, also referred to as xe2x80x9careal imagexe2x80x9d, is attributable to the light absorption and light scattering that occur in the resist in the course of exposure. Because the photoresist absorbs a certain fraction of the incident radiation, the observed radiative intensity decreases with increasing layer thickness in the photoresist. Consequently, those areas of the photoresist layer that are close to the surface are more greatly exposed to the light. In the case of a positive resist, therefore, the areas close to the surface are more readily soluble than the areas remote from the surface. The difference in solubility within an exposed area of the resist often leads to flattening and poor definition of the profile sidewalls in the case of positive resists. The light intensity profile in the photoresist, therefore, describes the distribution of a photochemically changed species: for example, in the case of a positive resist, the distribution of the photolytically generated acid in the photoresist.
In the case of negative resists, the decrease in radiative intensity with increasing layer thickness leads in the photoresist to greater crosslinking in the areas close to the surface, which, therefore, have a lower solubility than the areas remote from the surface. In the course of subsequent developing of the exposed photoresist, therefore, those areas of the photoresist layer that are remote from the surface are dissolved to a greater extent than the near-surface areas that lie above them, thereby likewise impairing the quality of the profile sidewalls and, hence, the resolution.
For the structuring transfer of the photomask into the underlying layer that is to be structured, the quality and the steepness of the resist profiles are of critical importance. One prior art approach to improving the quality of resist profiles in positive resists is described in European patent application EP 0 962 825 A1, corresponding to U.S. patent application Publication Ser. No. US 2002/0012867 A1 to Yasuda. There, improved steepness of the resist sidewalls is achieved by adding to the photoresist two photochemically active additives that are activated by radiation in respectively different wavelength ranges. On one hand, the photoresist includes a photoacid generator that, as already described above, reacts to exposure to light of a defined wavelength range by releasing an acid that then catalyzes the reaction of the convertible apolar groups of the layer-forming photoresist polymer to carboxylic acid groups and so brings about solubility of the photoresist in the polar developer. On the other hand, the photoresist includes, as a second photochemical additive, a crosslinking reagent that brings about a reduction in the solubility of the photoresist. The crosslinking reagent is likewise activated by radiation, with the radiation used for this purpose differing from the radiation used to activate the photoacid generator.
In a first patterning exposure step, according to Yasuda, the photoacid generator is activated in the areas determined by the mask layout. In a subsequent, second, floodlight exposure step, the entire photoresist layer is exposed without the use of a photomask and, hence, the crosslinking reagent is photochemically activated over the entire area of the photoresist layer. As a result of the chemical crosslinking of the photoresist that this initiates, its solubility is reduced. Because those areas of the photoresist close to the surface are more greatly exposed, they are more highly crosslinked and, hence, less soluble than the areas remote from the surface. Such a change in solubility acts in opposition to the change in solubility achieved in the first exposure step. Whereas, as a result of the first exposure step, the exposed areas close to the surface have a solubility that is increased relative to that of the areas remote from the surface, the second exposure step results in precisely the opposite gradient. Such selective solubility modification in the photoresist brings about increased developer selectivity in the aqueous developer, resulting, in turn, in steeper resist profile sidewalls.
Nevertheless, such an approach has one critical disadvantage: the crosslinking reaction, particularly in the near-surface areas of the photoresist, leads to the formation of a three-dimensional network polymer. The network polymer has a development behavior that has been changed relative to that of the original, linear, layer-forming polymer, thereby leading to xe2x80x9croughxe2x80x9d, i.e., imprecisely definedxe2x80x94frayed, for examplexe2x80x94profile sidewalls. Such roughness hinders the downstream process steps, such as substrate etching, for example.
It is accordingly an object of the invention to provide a process for structuring a photoresist layer that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that reduces or completely prevents the disadvantages described above. In particular, the present invention provides a process by which high transfer accuracy of the structure predetermined by the lithography mask into a photoresist layer is achieved.